This invention relates to a method for molding radial leaded electrical components and more particularly to novel steps for using knock-out pins for centering the component in the mold cavity.
Knock-out pins have been employed for many years to center a radial leaded electrical part in a mold during transfer molding. The rationale underlying this conventional centering method is based upon the fact that the component is supported by its leads that extend in one direction from the component body and the supposition that such a flimsily supported body will likely be deflected from its initial position by hot molding resin as it is introduced into the mold cavity. The likely directions of deflection are at right angles to the plane of the two or more radially extending leads. Two retractable coaxial knock-out pins located in the mold on opposite faces of the component are designed to extend into the cavity in directions at right angles to the leads so that they are capable of sandwiching the component during a first stage of filling the cavity with hot resin. In a second stage while the molding resin is still flowable, the pins are withdrawn from the cavity to a flush position with the cavity walls and the pressurized still flowable resin there fills the resulting voids. However, when this centering system is used in multi-cavity molds, a few of the molded components still tend to exhibit "show-throughs," i.e. the resin does not completely fill the mold cavity or the residual stresses in the component leads cause the component body to spring off center so that a portion of the component body shows through. Furthermore, the percentage of units exhibiting a show-through goes up when larger molds with more cavities are used. Thus attempts to increase the productivity of a molding press by increasing the mold capacity are quickly frustrated.
It is accordingly an object of this invention to provide a method for centering and during molding maintaining centered each of a number of radially leaded components in a cavity of a large capacity mold.